Military sonars are used for target detection, localization, and classification. They generally cover a
broader frequency range with higher source levels than civilian sonars. They are operated during both
training exercises and combat operations. Because far more time is spent in training than in combat,
training exercises may be the primary context in which marine mammals are exposed to military sonar.
Low Frequency Active (LFA) sonars are used for broad-scale surveillance; they are designed to allow
submarine tracking over scales of many hundreds to thousands of kilometers. Specialized support ships are
used to deploy LFA sonars, which consist of arrays of source elements suspended vertically below the ship.
The U.S. Navy’s Surveillance Towed Array Sensor System (SURTASS) LFA sonar uses an array of 18
projectors operating in the frequency range of 100 to 500 Hz, with 215 dB re 1µPa @ 1 m source level for
each projector (Johnson 2002). These systems are designed to project beams of energy in a horizontal
direction. The effective source level of an LFA array, when viewed in the horizontal direction, can be 235
dB re 1µPa @ 1 m or higher. The signal includes both continuous-wave (CW) and frequency-modulated
(FM) components with a bandwidth of approximately 30 Hz. A ping sequence can last 6 to 100 seconds,
with a time between pings of 6 to 15 minutes and a typical duty cycle of 10 to 15 percent. Signal
transmissions are emitted in patterned sequences that may last for days or weeks